This circuit was found in a Texas Instruments documentation from the 1980s. The circuit monitors engine temperature by warning of engine warm-up and warning when the fan is on.
This circuit is based on a 5 V LED flasher. We found it in October 1980 Radio Electronics...
With this system you can disconnect phone lines in case of need for protection. The circuit is...
This circuit was found in a Popular Electronics from the 90s. The circuit has 4 frequency ranges...